This invention relates to semiconductor wafer testing, and more particularly to determining the composition of a dielectric layer.
Thin dielectric layers (e.g., less than about 10 nm thick, such as about 2.0 nm thick or less) are widely used in semiconductor devices that are the building blocks of integrated circuits. For example, a thin dielectric typically separates a gate electrode from a channel region in a field effect transistor (FET).
The ever-shrinking dimensions of semiconductor devices demand increasingly thin dielectric layers. Presently, advanced devices use dielectric layers with a 1.3 nm effective thickness, however, industry analysts expect that devices will use 1.0 nm thick dielectric layers in 2006, and 0.5 nm in 2014 (see International Technology Roadmap for Semiconductors 2002 Update, available at the website public.itrs.net).
Dielectric layers are commonly formed from silicon dioxide (SiO2), which can exhibit large leakage current when the layers are very thin. Such leakage is undesirable in most semiconductor devices. In FET""s, for example, thin dielectric layers with large leakage currents could cause computer microprocessors to overheat and/or batteries in portable electronic equipment (e.g., notebook computers, PDA""s, and mobile phones) to drain rapidly.
One solution to leakage problems associated with thin dielectric layers is to replace SiO2 with other dielectric materials, called high-k dielectrics, which have a higher dielectric constant than SiO2. Candidate high-k dielectrics that show promise in applications demanding thin dielectric layers include mixed dielectrics, such as xe2x80x9coxynitrides,xe2x80x9d which include silicon, oxygen, and nitrogen. In order to reliably manufacture high-k mixed dielectric layers at high production yields, manufacturers will most likely desire metrology methods compatible with their manufacturing techniques that can accurately and rapidly characterize the layers.
In general, in a first aspect, the invention features a method for determining a composition of a dielectric layer on a semiconductor substrate, which includes monitoring a voltage across the dielectric layer under conditions where substantial leakage current flows across the dielectric layer, determining a leakage voltage for the dielectric layer from the monitored voltage, and determining the composition of the dielectric layer by comparing the leakage voltage to a reference voltage corresponding to the leakage voltage of a dielectric layer of known composition.
Implementations of the method may include one or more of the following features and/or features of other aspects.
The conditions where substantial leakage current flows across the dielectric layer can be achieved by depositing an electric charge on a surface of the dielectric layer using a corona discharge. The voltage across the dielectric layer can be monitored using a vibrating probe placed in proximity to the surface of the dielectric layer.
The dielectric layer can include first and second component materials, and the voltage across the dielectric layer can be monitored for a polarity at which current-voltage characteristics for the first and second component materials differ the most.
In general, in another aspect, the invention features a method for determining a composition of a test dielectric layer on a semiconductor substrate, which includes measuring a leakage voltage, VT, at a first polarity for the test dielectric layer, comparing VT to a reference leakage voltage, VR, corresponding to a leakage voltage at the first polarity for a reference dielectric layer having the same thickness as the test dielectric layer, wherein the reference dielectric layer comprises substantially none of a first material, and determining a value, XT, indicative of a concentration of the first material in the test dielectric layer based on a relationship between VT and VR.
Implementations of the method may include one or more of the following features and/or features of other aspects.
The method can include determining the thickness of the test dielectric layer. The method can also include determining the reference leakage voltage from the thickness of the test dielectric layer. Determining the thickness of the test dielectric layer can include measuring a leakage voltage, VT2, at a second polarity opposite the first polarity, proportional to the test dielectric layer thickness. The thickness of the test dielectric layer, T, can be determined according to the equation
T=(VT2xe2x88x92BR2)/AR2, 
wherein BR2 and AR2 are predetermined parameters relating a reference leakage voltage, VR2, corresponding to a leakage voltage at the second polarity for a reference dielectric layer comprising substantially none of the first material to a thickness of the reference dielectric layer, TR. In some embodiments, VR2=AR2xc3x97TR+BR2.
The reference dielectric layer can include a reference dielectric material having a conduction band energy, ERC, and a valence band energy, ERV, and the first material can have a conduction band energy, ETC, and a valence band energy, ETV, and wherein measuring VT includes selecting the first polarity based on ERC, ERV, ETC, and ETV. The first polarity can be negative when
|ERCxe2x88x92ETC| less than |ERVxe2x88x92ETV|. 
The first polarity can be positive when
ERCxe2x88x92ETC| greater than |ERVxe2x88x92ETV|. 
Measuring VT can include depositing an ionic charge having the first polarity onto a surface of the test dielectric layer in an amount sufficient to cause a measurable leakage current to flow across the test dielectric layer, monitoring a voltage of the dielectric layer after depositing the ionic charge, and determining VT based on the monitored voltage.
XT can be proportional to a difference between VT and VR. For example, XT can be determined according to the formula
XT=(VTxe2x88x92VR)/(VT2xe2x88x92BR2), 
wherein VT2 is a leakage voltage of the test dielectric layer at a second polarity opposite the first polarity and BR2 is a predetermined parameter relating a reference leakage voltage, VR2, corresponding to a leakage voltage at the second polarity for a reference dielectric layer comprising substantially none of the first material to a thickness of the reference dielectric layer, TR.
The method can also include calculating the concentration, [X], of the first material in the test dielectric layer from XT. [X] can be calculated according to the formula
[X]=CCALxc3x97XT+DCAL, 
wherein CCAL and DCAL are predetermined parameters relating [X] to XT.
The first material can include nitrogen. The reference dielectric layer can include SiO2.
In general, in a further aspect, the invention features a method for determining a composition of a test dielectric layer on a semiconductor substrate, which includes depositing an ionic charge of a first polarity onto a surface of the dielectric layer using a corona discharge, monitoring a voltage of the dielectric layer with a non-contact probe after depositing the ionic charge, determining a leakage voltage, VT, for the test dielectric layer based on the monitored voltage, and calculating a value, XT, indicative of a concentration of a first material in the test dielectric layer based on a difference between VT and a reference leakage voltage, VR.
Implementations of the method can include one or more of the following features and/or features of other aspects.
VR can correspond to a leakage voltage at the first polarity for a reference dielectric layer having the same thickness as the test dielectric layer, wherein the reference dielectric layer comprises substantially none of the first material.
Calculating XT can include first determining VR from a thickness of the test dielectric layer. The thickness of the test dielectric layer can be determined by measuring a leakage voltage, VT2, of the test dielectric layer for a second polarity opposite the first polarity, and calculating the thickness of the test dielectric layer using a function relating the thickness of a dielectric layer to its leakage voltage for the second polarity.
Embodiments of the invention can include one or more of the following advantages. Methods disclosed herein can be used to rapidly determine the composition of mixed dielectric layers (e.g., in a matter of minutes or less). Embodiments include non-contact methods for determining dielectric composition. These methods can utilize established corona charging and contact potential difference monitoring techniques and equipment. Extremely thin (e.g., less than about 5 nm, 3 nm, 2 nm, such as 1.3 nm or less) dielectric layers can be characterized, even in the presence of substantial leakage current using kinetic techniques for measuring leakage voltage. Static leakage voltage measurements can also be used.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.